Which of the following best describes how a column prefix index works and why its an advantage?

Which of the following best describes how a column prefix index works and why its an advantage?

A.
A Column prefix allows for a shorter index, making it read less from disk,

B.
A Column prefix allows for a shorter index name, making less code to be written.

C.
A Column prefix is another name for a left-most prefix, which is usefully with multi-column indexes.

Explanation:
Column prefix only uses the first n characters of the column as index, so smaller index to be read on lookup.